The monarchical system dictates that the citizens of Britain, Canada, Australia and many another Commonwealth country must pick their next head of state from a shortlist of one. Will the guarantee that the king will compensate for the privilege of belonging to this shrunken gene pool by keeping out of politics hold in the reign of Charles III? Or will we find that the promise of impartiality has gone?

There was a time when establishment voices could answer these questions with ease. Only leftwing conspiracy theorists thought the House of Windsor wielded political power, they said. Its role was to delight tourists and credulous natives with its hatches, matches and dispatches. But it never interfered in politics, which was why we enjoyed stable government and civil peace, unlike those unfortunate continentals with their dangerous republics. Although the official line was not accurate – George VI and the late Queen Mother shamed themselves with their support for the Chamberlain government's appeasement of Nazism – it was accurate enough for propaganda purposes.

The strange and bombastic figure of Charles Windsor is now making a nonsense of Britain's former certainties. Many and varied are the ways in which he deforms those around him. In the case of the law officers, he has turned the attorney general into a walking oxymoron.

Last week, Dominic Grieve MP QC rushed to please His Royal Highness by intervening in the due process of law. He blocked a decision by the Upper Tribunal of the Administrative Appeals Chamber to allow the public to read about Prince Charles's attempts to influence politicians and civil servants. At first, Grieve pretended that the Britain we used to know was still here. "Within a constitutional monarchy, where the sovereign is head of state but political power is exercised through a democratically elected government, the sovereign cannot be seen to favour one political party above another or to engage in political controversy." Which was fair enough. In a phrase that may return to haunt him, Grieve continued: "Inherited monarchy could not be preserved" if the sovereign abandoned political neutrality. And that ought to be true too.

If what he said were true now, however, there would be no difficulty in publishing Prince Charles's letters. They would be so dull – so filled with formal greetings and polite nothings – they would weary even the most obsessive collector of royal trivia. But without pausing for breath, or showing any embarrassment at the contradiction in his reasoning, the attorney general made it plain that Prince Charles's letters were anything but boring. They contained his "deeply held personal views and beliefs", he said. Included were "particularly frank" remarks about public affairs, which would make politicians unwilling to "engage" with him in future.

If you think Grieve makes Prince Charles sound like a lobbyist seeking special treatment for his favoured causes, you would be right. Allowing the public to see the letters of a prince it pays for and must accept as its next head of state without a democratic vote would, the attorney decided, "potentially have undermined his position of political neutrality". One second, the law officer says the royals must be neutral. The next, he admits that Britain's next monarch is not neutral and the state must censor to prevent the public learning this uncomfortable fact.

The tribunal Grieve overruled was more honest. Reading the judgment of Mr Justice Walker and his colleagues, one senses their mounting shock at how the prince has broken the conventions that kept the British state functioning. The tribunal did not order the release of genuinely private correspondence. On the contrary, it wanted the public to know only about the prince's lobbying. In this, he is as tireless as the British Bankers' Association or Unison. The prince considered himself entitled to use his access to ministers in order to "drive forward charities and promote views", the judges explained. They accepted that constitutional convention allows the heir to the throne to meet politicians and understand the workings of the state as part of his preparation to be king. But the prince was engaged in a "massive extension" of lobbying that went far beyond precedent and convention. It was a fundamental "point of principle" that lobbying by the heir to the throne "cannot have constitutional status", the tribunal concluded. Therefore, his future subjects should know what interests and causes the prince was pursuing in private.

My Guardian colleague Rob Evans asked officials to release the records. Tellingly, however, the tribunal was so concerned that on its own initiative, and without any prompting from the press, it decided to collect everything that was publicly known about the prince's political meddling. Now we have it all in one place. The rantings about architecture, which meant that by the early 1990s the Telegraph was complaining that the prince had an effective veto on new projects; the reactionary attacks on the Human Rights Act, which forced a Labour lord chancellor to tell him: "I do not see how discouraging people from enforcing their rights could result in a more responsible society"; the ultra-reactionary embrace of every variety of quackery and crankiness that led to his campaigns against foods that might feed the hungry and medicines that might heal the sick; and the babbling mysticism that produced his declaration that humanity took a wrong turn when Galileo showed that the Earth went round the sun.

I could go on, but when we have before us a prince who has not come to terms with the Renaissance, let alone the Enlightenment, the point is surely made. The danger in every hereditary system is that the nepotistic principle will eventually throw up a man wholly unsuited for the role tradition assigns him. For Britain's constitutional monarchy, Charles Windsor is that man. When Jonathan Dimbleby interviewed him on bended knee in the 1990s, and in more recent interviews with Vanity Fair, Charles stated that he does not intend to abide by the convention that he should stay out of politics. If his parents wanted him to rise above politics when he became king "that's their bad luck".

It is not republicans who should worry about his coronation – as I have said before, we can't wait – but monarchists. One day, maybe soon, they will have to explain to an embarrassed British public and a mocking world why they went to such lengths to censor the unavoidable truth that nine out of 10 people you pass in the street would make a better constitutional monarch than HRH, Prince of Wales.